The present invention relates generally to control systems, and more particularly to detecting and controlling the speed of a pulse-width modulated fan for temperature controlled systems.
Using pulse-width modulation (PWM) to control the rotational speed of a cooling fan is an established method for reducing the cost of a variable speed fan in a cooling system. Pulse-width modulating the power supply signal of the cooling fan can be used to vary the speed of a cooling fan. The speed of the fan is determined by the duty cycle of the power supply signal. For example, applying power having a duty cycle of 100 percent results in the fan operating at its nominally highest rate. Likewise, applying a power signal having a duty cycle of 50 percent generally results in a fan speed that is approximately half of its highest rate. A switch is used to actuate the fan by modulating the power supply. An N-type switching transistor is typically used to modulate the power supply signal because of its low cost. The N-type switching transistor is typically coupled in series between the fan motor and ground. A P-type switch, if used, would typically be coupled between the power supply and the fan.
The present invention is directed towards a control system that pulse-width modulates the duty cycle of a power supply signal that is used to control the rotational speed of a fan in a temperature-controlled system. The control system includes an edge detector for monitoring a tachometer signal that is provided by the fan. The tachometer signal is monitored at a rate (e.g., one Hertz) that is substantially lower than the rotational speed of the fan. The control system also includes a timer for recording time intervals between selected edges of a monitored tachometer signal. The speed of the fan is determined from the recorded time intervals. The control system extends the duty cycle of the pulse-width modulated signal to ensure that the fan power remains on over the period in which the tachometer signal is to be monitored. Maintaining fan power ensures the validity of the tachometer signal during the period that the fan power is maintained. The timer is also used to ensure that the pulse-width modulated signal is not extended beyond the time for which a recording of a tachometer signal for a properly operating fan can be made. This ensures that the extended PWM pulse is not long enough to substantially affect the fan speed.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detailed description of illustrated embodiments of the invention, and to the appended claims.